1. Field of the Invention
The present invention relates to wireless communications, and more specifically, to an apparatus and method for reporting a power headroom regarding a component carrier in a wireless communication system.
2. Related Art
The LTE (Long Term Evolution) system, a post wireless communication system, has recently taken its step into the business world. The state-of-art communication system is rapidly spreading with the increasing demand for support of high-quality voice and mass data services while ensuring users' mobility. This next-generation system offers low transmission delay, high transmission rate and system capacity, and increased coverage.
To the end, the LTE system adopts carrier aggregation (CA). Carrier aggregation is also referred to as spectrum aggregation or bandwidth aggregation, and this scheme aggregates multiple component carriers to enable broadband transmission and reception of data. Each aggregated carrier is referred to as a component carrier (CC). Each component carrier is defined with a bandwidth and a center frequency.
For a high transmission rate, communication services require an increased bandwidth. In the case of low QoS services, allocation of a large bandwidth to them may cause a huge power waste. Meanwhile, base stations may make efficient use of resources of a user equipment (UE) with the power information from the UE. Power control is a core technique for base stations to reduce the UE's power consumption while minimizing interference for an efficient resource distribution. To that end, the UE may determine uplink transmission power depending on scheduling information provided from the UE to base stations, such as bandwidth, modulation and coding scheme (MCS), or transmit power control (TPC).
Adoption of multiple component carriers requires comprehensive consideration of uplink transmit power of each component carrier, and the UE's power control is thus encountered with more complicated situations. This may put the UE at issues in light of maximum transmit power. A maximum transmit power refers to the upper cap within an allowable transmit power range of a UE. Typically, a UE should be operated at lower power than its maximum transmit power. In case a schedule prepared by a base station requires a UE to use a higher transmit power than the UE's maximum transmit power, the actual uplink transmit power may exceed the maximum transmit power.
Accordingly, the information on the uplink transmit power needs to be shared between the UE and the base station. In particular, a need exists for a delicate signaling scheme for a power headroom in order to efficiently adjust transmit power.